Generally, in the process of making a semiconductor device, a flash EEPROM (Electrically Erasable Read Only Memory) cell having both functions of electrical programming and erasing may be classified into a stack-gate structure and a split-gate structure.
As shown in FIG. 1A, a conventional flash EEPROM cell of stack-gate type has a structure in which a tunnel oxide film 5, a floating gate 6, an interpoly oxide film 11 and a control gate 12 are sequentially stacked on a silicon substrate 1 between a drain region 7 and a source region 8.
As shown in FIG. 1B, a conventional flash EEPROM cell of split-gate type has a structure in which a tunnel oxide film 5, a floating gate 6, an interpoly oxide film 11 and a control gate 12 are sequentially formed on a silicon substrate 1 between a drain region 7 and a source region 8, it has a stacked structure on the drain region's side 7 and it has a structure in which the control gate 12 comprising the upper layer of the stacked structure extends toward the source region 8. The silicon substrate 1 underlying the extended control gate 12 becomes a select gate channel region 9.
Though the stack-gate structure has an advantage in accomplishing a high density of the device because it can reduce the area per cell compared with the split-gate structure, it however has a disadvantage in that it is over-erased when being erased. Whereas, though the split-gate structure can overcome the disadvantage of the stack-gate structure, it has a disadvantage in accomplishing a high density of the device because it can not reduce the area per cell compared with the stack-gate.
On the one hand, the flash EEPROM cell of stack-gate type or split-gate type may perform the functions of programming and erasing when a high voltage is applied to the cell. When performing programming and erasing functions using a high voltage, a band-to-band tunneling and a secondary hot carrier are generated due to a strong electric field formed at the overlap region between a junction region and a gate electrode. However, the tunnel oxide film is degraded due to generation of the band-to-band tunneling and the secondary hot carrier because the tunnel oxide film of the cell is usually formed thin in thickness of about 100.ANG., thereby reducing the reliability of the device.